Upper plate deformation as marker for the Northern STEP fault of the
Ionian slab (Tyrrhenian Sea, central Mediterranean)
Alfonsa Milia
a,⁎
, Pietro Iannace
b, Magdala Tesauro
c, Maurizio M. Torrente
ba
IAMC, CNR, Calata Porta di Massa, Porto di Napoli, I-80100 Naples, Italy
bDST, Università del Sannio, Via dei Mulini 59/A, I-82100 Benevento, Italy c
Utrecht University, Budapestlaan, 6, 3584 CD Utrecht, The Netherlands
a b s t r a c t
a r t i c l e i n f o
Article history: Received 28 January 2016
Received in revised form 19 August 2016 Accepted 22 August 2016
Available online 24 August 2016
The Eastern Tyrrhenian margin (ETM), the active boundary of the Tyrrhenian Sea backarc basin, is the key for un-derstanding the geodynamics of the central Mediterranean. Numerous seismic tomography studies have been carried out in this region, proposing different reconstructions of the lower subducting plate and cause of the slab-break-off existing beneath the Southern Apennines. However, the area and mode of the recent deformation of the Tyrrhenian Sea are still not fully defined and understood. In this study, we combine the analysis of a recent seismic tomography model and geological data, in order to understand the relationship between the subducting lower plate and the tectonic evolution of the sedimentary basins formed on the upper plate.
With this aim, we interpreted a large data set of seismic reflection profiles and several well logs. The results con-sist in 2D and 3D geological models of the basins, sedimentary infill, and fault networks. Taking into account the geological data of the ETM and those of the adjacent innerflank of the Apennines, we observe: (i) a system of linked sedimentary basins developed on a narrow deformation belt bounded by transform fault zones; (ii) a poly-phase rifting within the upper plate; (iii) an abrupt change of the direction of extension (~90°), from NE-oriented in the Lower Pleistocene to SE-oriented in the Middle Pleistocene. Since these ETM features are not the typical expressions of the current backarc extensional models, we propose a link between the evolution of upper plate and the onset and development of a STEP (Subduction-Transform-Edge-Propagator) fault along the northern margin of the Ionian slab.
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Keywords: Backarc extension Tectono-stratigraphy 3D visualization STEP fault Tyrrhenian Sea 1. Introduction
Extension in backarc basins plays a central role in the tectonic evolu-tion of many areas of the Earth (e.g.Schellart and Lister, 2005; Cloetingh et al., 2013, 2015). Backarc environments are characterized by rift basins developing in convergent plate boundaries on the concave side of arcs. They are widely spread around the globe with numerous examples in the western Pacific and Mediterranean Sea (e.g. Taylor, 1995; Faccenna et al., 1996). The Tyrrhenian Sea is the youngest backarc basin of the Western Mediterranean linked to development of the Ioni-an subduction zone (Fig. 1). The Eastern margin of the Tyrrhenian Sea (Eastern Tyrrhenian Margin, ETM) is characterized by the opening of the youngest sedimentary basins and fault activity. Studying the tecton-ic Quaternary evolution of the ETM greatly improves our understanding of the geodynamics of the Central Mediterranean, since it records the mode of extension of a backarc migration toward the continental area and is located in correspondence of the northern boundary of the Ionian
subducting plate. Such a study makes it also possible to identify the causes of the present-day tectonics, which is of primary societal interest, as this region is densely populated. Furthermore, the ETM is character-ized by recent volcanic activity (e.g., Vesuvius and Campi Flegrei) and thus has a large potential of geo-resources, such as mineral deposits and geothermal energy (e.g.De Vivo et al., 1989; Serri, 1990; Bodnar et al., 2007; Lima et al., 2009; Milia, 2010; Milia and Torrente, 2011; Carlino et al., 2012).
Seismic tomography studies of the central Mediterranean (e.g.
Wortel and Spakman, 2000) revealed an extended high-velocity zone interpreted as the subducting Adriatic and Ionian slab. The continuity of the Adriatic slab is interrupted, as observed form the low velocity zones located in the uppermost part of the mantle underlying the south-ern Apennines. The existence of a gap in the structure of the subducted slab, interpreted as a slab detachment, was postulated already in 1971 by Isacks and Molnar (Isacks and Molnar, 1971). Afterwards,Wortel and Spakman (2000)andGovers and Wortel (2005)hypothesized a lat-eral migration of the slab detachment due to tears formation. These au-thors suggested that the slab edges correspond to Subduction Transform Edge Propagator (STEP) faults, characterized by a very specif-ic time-space evolution in the upper plate deformation.Govers and
⁎ Corresponding author.
E-mail addresses:alfonsa.milia@iamc.cnr.it(A. Milia),piannace@unisannio.it
(P. Iannace),m.tesauro@uu.nl(M. Tesauro),torrente@unisannio.it(M.M. Torrente).
http://dx.doi.org/10.1016/j.tecto.2016.08.017
0040-1951/© 2016 Elsevier B.V. All rights reserved.
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Tectonophysics
Wortel (2005)hypothesize the presence of STEP fault as boundary of narrow subducting oceanic plates locked by continental crust. One of these examples is in the Mediterranean region, where the Ionian slab may have two STEP faults in the northern and southern parts. In this case whereas the southern STEP fault is well represented by the E-W boundary between the Africa and backarc margin (e.g.Carminati et al., 1998; Gvirtzman and Nur, 1999; Argnani, 2009), the deformation zone linked to the northern STEP fault of the Ionian slab is poorly constrained (e.g.Gvirtzman and Nur, 1999; Marani and Trua, 2002; Govers and Wortel, 2005; Lucente et al., 2006; Rosenbaum et al., 2008). The stratigraphic and tectonic study of the sedimentary basins locat-ed on the upper plate provides constraints on the timing and duration of the rifting phases and overall geometry of rift basins that evolve in re-sponse to the subducting slab dynamics. Within this frame, the analysis of this sedimentary basin gives fundamental constraints on the recon-struction of the geodynamic evolution and clarifies the relationships be-tween deep and shallow structures in convergent regions.
The development of software for 3D modeling has opened a new frontier in Earth Sciences, leading to a more accurate spatial analysis
of geological structure and to 3D models. Numerous papers deal with the integration of different kind of data for a 3D reconstruction of sub-surface structures at a regional scale (e.g., De Donatis, 2001; D'Ambrogi et al., 2004; Dhont et al., 2005; Smith, 2005; Bigi et al., 2013). The subsurface data generally used for 3D reconstructions are the seismic data, integrated with well data logs. Several deep sedimen-tary basins that are present along the Eastern Tyrrhenian Margin have been studied separately (e.g.,Sacchi et al., 1994; Milia, 1999; Milia et al., 2009, 2013) and thus a synthesis of the relationships among each stratigraphic unit between these extensional basins and a complete analysis of the fault pattern is lacking so far. In this study we construct a 3D model, providing the detailed structure of the Tyrrhenian margin, impossible to obtain with only 2D seismic dataset. The results obtained by integrating multi-source data (seismic reflection profiles and bore-holes), greatly improves our understanding of the geological structures of the study area.
The analysis of the crustal deformation of the Southern Campania Margin, coupled with a review of the Northern Campania Margin ba-sins and Paola Basin makes it possible to identify: a) the area of the
youngest deformation of the Tyrrhenian Sea; b) the genetic link be-tween the deep and rapidly evolving sedimentary basins; c) the ki-nematic evolution of the study region. In addition, using the results of this analysis and of the recent regional seismic tomography model ofKoulakov et al. (2009), we propose a model of the tectonic evolution of the area,finding a link between the shallow and deep structures.
2. Geologic framework
The Western Mediterranean is characterized by a complex pattern of backarc basins in response to migration of the Calabria subduction zone (Dewey et al., 1989; Mauffret et al., 2004). The formation of these backarc basins starts with the opening of the Liguro-Provencal basin, be-tween 30 and 16 Ma (Cherchi and Montadert, 1982; Burrus, 1984; Séranne, 1999), followed by the Algerian Basin, between 16 and 7 Ma (van Hinsbergen et al., 2014), andfinally the Tyrrhenian basin, between 12 Ma and the Present (e.g.Kastens et al., 1987; Sartori, 1990; Milia and Torrente, 2014).
The tomographic studies of the deep structure of the Tyrrhenian-Ap-ennine system and the distribution of subcrustal earthquakes (e.g.
Frepoli et al., 1996; Anderson and Jackson, 1987; Cimini and De Gori, 2001) reveal the occurrence in the southern Tyrrhenian sea of a Benioff zone, associated to the subduction of the Ionian plate below Calabria, that dips northwestwards from the surface down to 500 km. Further-more, it has been suggested that a rollback subduction of the Calabrian slab induced a local scale mantleflow parallel to the trench (Gvirtzman and Nur, 1999; Civello and Margheriti, 2004; Faccenna et al., 2007).
The geophysical data reveal that the Southern Apennine fold-and-thrust belt covers the autochthonous Apulian continental crust, 25– 30 km thick (Mostardini and Merlini, 1986; Menardi-Noguera and Rea, 2000; Roure et al., 1990). In contrast, the Calabria terrane overthrusts the oceanic Ionian crust, considered a late Paleozoic-Early Mesozoic oceanic embayment of the Neothetys (Catalano et al., 2001; Stampfli and Borel, 2004). The oceanic nature of the Ionian crust has been also confirmed by seismic refraction data (de Voogd et al., 1992). The Tyrrhenian Sea, at the rear of the Apennine fold-and-thrust belt is a typical rift basin, characterized by a thinned lithosphere (b30 km) and continental crust, with a Moho depthb 10 km in the bathyal plain and ~ 20 km along its Eastern margin (Ferrucci et al., 1989; Nicolich, 1989; Panza, 1984), numerous normal faults, large Bouguer anomalies (N250 mGal in the bathyal plain;Mongelli et al., 1975), and high heat flow values (up to 200 mW m−2;Della Vedova et al., 2001). The features
recognized in the bathyal area also occur in the Campania Margin char-acterized by high heatflow values and shallow Moho reaching 10 km at Campi Flegrei (Ferrucci et al., 1989).
The hinge zone between the Southern Apennine/Calabria fold thrust belt and the bathyal area of the Tyrrhenian Sea corresponds to the East-ern Tyrrhenian Continental Margin, subdivided into NorthEast-ern Campania Margin (NCM), Southern Campania Margin (SCM), and Calabria Margin (CM) (Fig.1).
The NCM is characterized by the presence of up to 5 km deep basins filled by Quaternary clastic and volcaniclastic deposits (Ippolito et al., 1973; Rosi and Sbrana, 1987; Mariani and Prato, 1988; Milia, 1999; Milia et al., 2003). The NCM is affected by NE-SW, NW-SE, NNE-SSW and E-W faults. It started to form in the Lower Pleistocene and is charac-terized by the superposition of the three extensional events (Milia and Torrente, 1997, 1999, 2011, 2015b; Bellucci et al., 2006; Torrente et al., 2010; Milia et al., 2013).
The SCM has been studied by few authors who have investigated mainly the continental shelf area (Bartole et al., 1984; Sacchi et al., 1994; Casciello et al., 2004). Previous studies of Salerno Bay suggest that NE-trending normal faults controlled a thick depocenter (N2 km, Mina 1 well) of the basin (Bartole et al., 1984; Sacchi et al., 1994; Casciello et al., 2004). The age of these syn-rift deposits is controversial, Pliocene-Pleistocene according toBartole et al. (1984)andSacchi et al.
(1994)or Pleistocene according toCasciello et al. (2004). The Sele Plain, onshore extension of the Salerno Bay, is characterized by a Pleis-tocene clastic succession up to 1500 m-thick (Sele 1 well), showing at the base lower Pleistocene conglomerates (Eboli Fm;Cinque et al., 1988; Russo, 1990; Brancaccio et al., 1991), displaced by NW-SE and NE-SW normal faults (Gars and Lippmann, 1984; Zuppetta and Sava, 1992).
The CM is characterized by the Paola Basin, which has a very thick Plio-Quaternary succession (N4 km,Fabbri et al., 1981), affected by Qua-ternary folding (Argnani and Trincardi, 1988). Recently,Milia et al. (2009)proposed a poliphased tectonic evolution of the Paola Basin superimposed to a Serravallian E-W extension: Lower Pleistocene N-S extension (basin formation), followed NW-SE left lateral strike slip tec-tonics with associated folding, and Middle-Late Pleistocene NW-SE right-lateral strike slip tectonics (minor pull-apart basins).
Two main tectonic scenarios have been proposed for this backarc basin:
(1) An approximately E-W direction of extension, as suggested by fault patterns, which controlled the opening of extensional sedimentary basins since the Upper Miocene. This extension direction is orthogonal to Apennine foredeeps and coherent with the eastwards migration of the Adria-Apennine trench (Sartori, 1989; Patacca et al., 1990; Doglioni, 1991). The contemporaneous formation of the Tyrrhenian Sea and Apennine thrust belt migrating eastwards, leadPatacca et al. (1990)to propose a genetic link between the extensional and compres-sion tectonics (Fig. 1b). Extension in the Tyrrhenian Sea and shortening in the Apennine thrust belt, as a result of arc migration (rollback) driven by the sinking of the lithosphere, was the mechanism invoked by
Malinverno and Ryan (1986).Doglioni (1991), and more recently
Roure et al. (2012), interpreted the E-W opening of the Tyrrhenian Sea until the Pliocene/Lower Pleistocene and the contemporaneous shortening of the Apennines with a model of the West-dipping subduc-tion of the Adriatic-Ionian slab beneath the Tyrrhenian Sea. This model of evolution includes the presence of E-W transform faults across the Tyrrhenian Sea (Selli, 1981; Lavecchia, 1988; Rosenbaum and Lister, 2004; Finetti and Del Ben, 2005).
(2) Opening of the Tyrrhenian backarc basin toward SE, as indicated by the age of the oldest sediments drilled and by the sparse data of the DSDP and ODP in the bathyal area, linked to the rollback of the Ionian slab toward SE, as imaged by the tomographic models (e.g.Cavinato and De Celles, 1999; Gvirtzman and Nur, 1999; Wortel and Spakman, 2000; Faccenna et al., 2004, 2007; van Hinsbergen et al., 2014). Further-more, the entire southern Apennine thrust belt is characterized by a sys-tem of NW-SE trending left-lateral strike-slip fault zones post-Lower-Middle Pleistocene in age (Fig. 1b;Catalano et al., 1993, 2004; Cinque et al., 1993; Schiattarella et al., 2005). This geodynamic scenario of a NW-SE opening of the Tyrrhenian Sea is consistent with a NW-SE sinis-tral transform fault located on the eastern Tyrrhenian margin (Wortel and Spakman, 2000; Marani and Trua, 2002).
3. Data and methods
We constructed a 3-D subsurface model of the basin's basement of the eastern Tyrrhenian margin generated through the interpolation be-tween close and regularly spaced 2D seismic reflection profiles, inte-grated and calibrated with borehole data (Fig. 2). This work has been carried out using a seismic and borehole dataset and a Geographical In-formation System (GIS) software (Kingdom, IHS Inc.), which constructs a 3D representation of a geologic volume at depth. This study was car-ried out through the following steps: a) collection of all the available seismic profiles and boreholes data; b) implementation of a GIS geolog-ical data base; c) interpretation of the seismic profiles and calibration of the seismic unit using well-log data; d) construction of 2D and 3D models of the subsurface.
A total of 6000 km of seismic lines and data from 25 wells were col-lected in the study area (Fig. 2). We used seismic reflection profiles with different resolution and penetration: multichannel seismic profiles (ViDEPI, 2009), CROP seismic profiles (Scrocca et al., 2003) and Sparker data. Our study uses 1050 km of CROP offshore seismic sections of the eastern Tyrrhenian margin (Fig. 2). The Sparker seismic data set was ac-quired with a Multispot Extended Array System (MEAS). The output power of the MEAS, transmitted through a 36-tip array, was 16 kJ. Ver-tical recording scales were 2.0 s with a maximum verVer-tical resolution of 6 m. The data were subsequently processed in order to obtain a consis-tent dataset: seismic line basemaps and well position were geo-refer-enced in a common coordinate system (European Datum 50) and assembled in a dedicated GIS environment (Kingdom, IHS Inc.). Raster images of the overall seismic profiles were converted to segy format using image2segy, a free tool developed by Marcel Farran (Instituto de Cièncias del Mar., Barcelona University) for MATLAB software.
We interpreted the seismic data-set using the seismic stratigra-phy method: seismic units are groups of seismic reflections, whose parameters (configuration, amplitude, continuity, and frequency) differ from those of adjacent groups. Sedimentary units were delin-eated on the basis of contact relations and internal and external con-figurations (e.g. Mitchum et al., 1977). The seismic units were calibrated using the lithostratigraphic and chronostratigraphic data of offshore and onshore boreholes (Ippolito et al., 1973; Rosi and Sbrana, 1987; ViDEPI, 2009; Brocchini et al., 2001). Based on seismic facies, well data analysis and dated units, three unconformity-bounded units have been identified (Unit A, Unit B, Unit C) in the basin infills. Notably, we placed in our stratigraphic analysis the base of the Pleistocene at the base of the Gelasian stage (2.58 Ma), in agreement with the“Global chronostratigraphical correlation table for the last 2.7 million years v. 2010” (Cohen and Gibbard, 2010). Thus, the Gelasian succession of the Eastern Tyrrhenian mar-gin, previously attributed to the upper Pliocene, has been ascribed to the lower Pleistocene. Faults were interpreted on seismic reflection profiles, mapped in a GIS environment, and displayed as lines on structure contour maps and isochron maps. Due to a relatively close spacing between 2-D seismic lines (1–10 km in NCM, 3– 15 km in SCM, 5–10 km in CM), it was possible to recognize and link major faults based on their geometry, dip direction and amount of throw.
Gridding and contouring were performed on geological horizons to generate 2D models (contour maps) and isochron maps of the succes-sion. In order to select the best algorithm and processing parameters, it-erative testing was carried out. Thefinal step was the construction of the 3D digital model using the Vu-PACK module (Kingdom software) that
permits more accurate interpretations and controls opacity, color, and lighting of volumes, horizons and faults.
4. Stratigraphy
In order to define the stratigraphic setting of the Eastern Tyrrhenian Margin, we interpreted a large data set of seismic and wells collected in the Southern Campania Margin and revised the available geological data in the Northern Campania and Calabria margins. We mapped the pre-Quaternary basement and three stratigraphic units (A, B, C) of the basin infills for 350 km along the ETM, to establish a coherent chrono-stratigraphic framework across the sedimentary basins and to build a 3D model of the study area.
4.1. Basement
In the NCM, the acoustic basement features reflection-free and cha-otic seismic facies (Figs. 3, 4). It is composed of Meso-Cenozoic carbon-ate rocks forming the Apennine Platform unit of the thrust belt. These rocks have been drilled in the Mara, Apramo and Trecase wells, dredged offshore Naples Bay and outcrop in the mountains bounding the Campa-nia Plain and the Gaeta Bay (Fig. 2) (Milia, 1999; Milia and Torrente, 1999, 2015b; Milia and Torrente, 2011; Milia et al., 2013).
In the SCM, the acoustic basement corresponds to three seismic units calibrated by wells (Fig. 5): a reflection-free seismic unit bounded at the top by strong reflectors (Mz), a chaotic seismic facies unit (Lc), and a seismic unit with strong reflectors (Cg). Units Mz, Lc and Cg cor-respond, respectively, to Mesozoic rocks of the Apennine Platform unit, overthrusted by Cenozoic rocks of the Liguride basinal units (Nord-Calabrese and Parasicilide units;Bonardi et al., 1988; Mauro and Schiattarella, 1988; Bonardi et al., 2009; Vitale et al., 2011), uncon-formably overlain by the Cilento Group (Amore et al., 1988; Cavuoto et al., 2004). This basement has been encountered in the offshore wells (Mina 1, Milena 1 and Margherita Mare 1;Fig. 2), dredged in the Salerno through and the Issel ridge (Sartori, 2005), and crop out in the moun-tains bounding the Sele Plain (Fig. 2). The Liguride units are composed of highly deformed clays, marls, sandstones with some levels of lime-stones Paleocene-Lower Miocene in age (Lc), whose thickness increases from northwest to southeast. They are overlain by Langhian–Lower Tortonian clays, marls, conglomerates and chalky limestones of the Cilento Group (Cg).
The acoustic basement of the CM shows reflection-free seismic facies (Fig. 6) and is made up of high-grade metamorphic and intrusive rocks, Paleozoic in age. This basement is covered by Serravallian-Messinian deposits, characterized by high-amplitude and continuous reflectors, that record the oldest stages of the Paleo-Tyrrhenian opening (Milia et al., 2009; Milia and Torrente, 2014).
The contour map and the 3-D model of the basement (Figs. 7, 8), along the Eastern Tyrrhenian Margin, reveal a complex architecture due to the occurrence of several fault sets (NW-SE, NE-SW, E-W and NNE-SSW). Thus, this basement presents several structural depressions with associated basins up to 5/6 km-deep, from North to South: North-ern Gaeta Basin (NGB), Central Gaeta Basin (CGB), Campania Plain Basin (CPB), Southern Gaeta Basin (SGB), Campi Flegrei-Naples Bay Basin (CFNB), Salerno Bay Basin (SBB), Salerno Valley Basin (SVB), Salerno-Cilento Basin (SCB), Salerno-Cilento Basin (CB), Sapri Basin (SB) and Paola Basin (PB).
4.2. Unit A (Lower Pleistocene)
Unit A overlies the basement and has been dated Lower Pleistocene on the base of well logs stratigraphy, outcrop data and sequence stratig-raphy. It has been identified in the Gaeta Bay basins (Iannace et al., 2013; Milia et al., 2013), Campania Plain basin (Milia and Torrente, 2015b), Campi Flegrei-Naples Bay basin (Milia, 1999; Milia and Torrente, 1999, 2011; Milia et al., 2003) and Paola Basin (Milia et al., 2009).
In the NCM this unit is characterized by parallel reflectors with var-iable amplitude and frequency and very good continuity seismic facies
(Figs. 3, 4). This seismic facies corresponds to shallow-water marine de-posits (Milia, 1999; Milia et al., 2003; Milia and Torrente, 2015b). Unit A is characterized by an aggradational stacking pattern and its lower boundary corresponds to an onlap surface, whereas its upper boundary to an erosional one.
We identified unit A for the first time in the SCM. It occurs in a wide-spread area, covers unconformably the Cenozoic tectonic unit and is wedge-shaped (Fig. 5). Unit A is characterized by parallel and/or sub-parallel internal reflections configuration, with high to moderate ampli-tude, good to moderate continuity and an increase of reflectors frequen-cy upwards. Unit A, drilled in Mina 1 and Milena 1 wells (Fig. 2), has a
Fig. 3. Uninterpreted and interpreted seismic section crossing central and southern Gaeta, modified afterTorrente and Milia (2013). The seismic section displays the basement overlain by units A, B, and C and several normal faults. V0 corresponds to a lower Pleistocene volcano.
Fig. 4. Regional section through the Naples Bay-Campi Flegrei basin and seismic profile crossing Naples Bay. Mz = Mesozoic rocks forming the basement of the basin, A = Unit A, B = Unit B, C = Unit C, PV = Penta Palummo volcano.
Lower Pleistocene age (biozone MPL6) and is composed of silty clays, marly clays, marls interbedded withfine sands and polygenic conglom-erates, and abundance of carbonaceous materials. These deposits pass upwards to gravels interbedded with sandy clays, marly clays andfine sands and abundance of carbonaceous materials (Sele 1 and Mina 1 wells) Lower Pleistocene in age (biozone post-MPL6). According to our interpretation, Unit A was deposited in Southern Campania Margin between 2.3 and 0.7 Ma and its sedimentary facies changed from coastal shallow-water environment sediments to shelf environment. This strat-igraphic evolution witnesses a syn-sedimentary tectonic subsidence.
Unit A is made up of parallel reflectors with aggradational stacking pattern and reaches a thickness of 2.5 s in the CM (Fig. 6). The age of this unit, drilled in the Marta well (Fig. 2), is Pliocene-Lower Pleistocene. Its upper part displays a wedge-shaped external form thickening to-ward E/NE. Stratal architecture, growth fold geometry and slumps sug-gest a syn-sedimentary deformation. According to the sequence stratigraphic analysis ofMilia et al. (2009)the age of the syn-sedimen-tary wedge is 1.0–0.7 Ma.
The isochron map of the Unit A (Fig. 9) features seven thickness maxima (N1.5 s, twtt): four in the NCM (Northern Gaeta Basin and Cen-tral Gaeta Basin, Campania Plain, Campi Flegrei-Naples Bay basin); two in the SCM (Salerno Bay basin and Cilento basin) and one in the CM (Paola basin). These fault-bounded basins are characterized by high sediment supply and depositional rates that balanced the tectonic subsidence.
4.3. Unit B (Early Middle Pleistocene)
Unit B is the intermediate part of the Quaternary succession of the Eastern Tyrrhenian Margin and was dated 0.7–0.4 Ma using well logs stratigraphy, outcrop data, and sequence stratigraphy. It has been previ-ously identified in the NCM (Gaeta Bay basin,Iannace et al., 2013; Milia et al., 2013; Campania Plain basin,Milia and Torrente, 2015b; Campi
Flegrei-Naples Bay basin,Milia, 1999; Milia and Torrente, 1999, 2011; Milia et al., 2003) and CM (Paola Basin,Milia et al., 2009). Its stratal ar-chitecture, with the exception of the Paola Basin, shows progradational stacking patterns. In the Northern Gaeta Basin Unit B corresponds to a 0.8 s-thick sigmoid-oblique prograding complex that covers the sub-horizontal strata of Unit A, suggesting the infill of an undeformed basin. Instead, in the Campi Flegrei-Naples basin Unit B overlies an an-gular unconformity at the top of the tilted Unit A (Fig. 4). The stratal pat-tern, facies architecture, and thickness of unit B in the Campania Plain suggest a rapid tectonic subsidence of the area. The normal faults, that caused this basins subsidence, trend NE-SW.
In the CM the sediments coeval to Unit B reveal a dramatic change in the basinfill (Fig. 6): (i) basal onlap surface indicating an abrupt modi-fication in the stratigraphic architecture occurred at the 0.7 Ma se-quence boundary; (ii) distal stratal terminations of Unit B onlapping theflank of the fold and filling of the synformal basin; (iii) parallel seis-mic configuration indicating a horizontal filling of the basin as younger strata thin toward the coast suggesting an uplift of the coastal area.
We identified Unit B for the first time in the SCM, where is character-ized by a complex sigmoid-oblique progradational configuration with high to moderate amplitude, moderate frequency and good continuity reflections. This unit, drilled in the Sele 1, Mina 1 and Milena 1, Margherita Mare 1 wells, reposes on the sub-horizontal strata of the Unit A, indicating sediments aggradation/progradation from the coast during a relative tectonic stability of the area (Fig. 5). It is composed of gravels, sands, clays and marly clays Middle Pleistocene in age. In con-trast, in the deep basin, Unit B shows a parallel and/or sub-parallel inter-nal reflections configuration, with variable amplitude and frequency and good-continuity reflections (Fig. 10). This seismic facies indicates the deposition of pelagic/hemipelagic sediments interbedded with tur-bidites. The reflectors are convergent southeastwards, suggesting a fault controlled block tilting. The isochron map of the Unit B shows several thickness maxima (Fig. 11).
4.4. Unit C (Late Middle Pleistocene to present)
Unit C represents the upper part of the Quaternary succession of the Eastern Tyrrhenian Margin and was dated post-0.4 Ma on the base of well logs stratigraphy, outcrop data and sequence stratigraphy. It has been formerly identified in the NCM (Gaeta Bay basin,Iannace et al., 2013; Milia et al., 2013; Campania Plain basin,Milia and Torrente, 2015b; Campi Flegrei-Naples Bay basin, Milia, 1999; Milia and Torrente, 1999, 2011; Milia et al., 2003) and CM (Paola Basin,Milia et al., 2009). We recognized Unit C for thefirst time in the SCM.
Along the ETM Unit C displays two different architectures: prograding unitsfilling pre-existing accommodation space and syn-tec-tonic wedges related to different stages of subsidence. The Northern and Central Gaeta basins and the Salerno Bay and Salerno-Cilento basins are characterized by a thick regressive succession, featuring high-continuity reflectors with variable amplitude and progradational architecture. This seaward lateral migration of the sedimentary deposition that covers the sub-horizontal top of Unit A, indicates a period of relative tectonic sta-bility (Fig. 5).
In the Southern Gaeta Bay and Campania Plain basins a post-0.4 Ma thick succession of clastic deposits and volcanics (Fig. 3) was emplaced during fault activity (Bellucci et al., 2006; Milia et al., 2006; Torrente et al., 2010; Torrente and Milia, 2013; Milia and Torrente, 2015b). The iso-chron map of Unit C (Fig. 12) features two striking depocenters (N2 s, twtt)filled by voluminous volcanic units in the Southern Gaeta Bay
and Campi Flegrei basins. In contrast, others thickness maxima (up to 1 s, twtt), roughly parallel to the coast, exist in the Northern Gaeta Bay-Central Gaeta Bay area and offshore Cilento, reflecting the shelf margin of the coastal progradation. During the deposition of the Unit C the Paola Basin experienced the deposition of a relatively thick slope frontfill unit.
5. Tectonics
The interpretation of the seismic data reveals a complex fault pattern along the Eastern Tyrrhenian Sea due to a polyphase tectonic evolution. The 2-D and 3-D models of stratigraphic surfaces, faults and isochron maps of the stratigraphic units reveal that the major faults bounding the basins correspond to normal faults and strike-slip transfer faults (Figs. 7-9, 11-12). The process of lithospheric extension within the East-ern Tyrrhenian Sea produced tilted blocks/half grabens (sensu
Wernicke and Burchfiel, 1982). Active extensional faulting and sedi-mentation are intimately linked during basin evolution because tecton-ics controls the creation of the accommodation space, sediment supply and variation of the ratio between subsidence and sedimentation (White et al., 1986; Blair and Bilodeau, 1988; Schlische and Olsen, 1990; Muto and Steel, 1997). We recognized in the Eastern Tyrrhenian Sea positive inversion structures superimposed on previous extensional basins. Even if several studies (Cooper and Williams, 1989; Coward, 1994; Buchanan and Peter, 1995; Tavarnelli, 1996; Sandiford, 1999; Tavarnelli and Peacock, 1999; Tavarnelli et al., 2001; Turner and Williams, 2004; De Paola et al., 2005; Henk and Nemčok, 2008) claim that the positive inversion structure involves the reversal of extensional fault movement during contractional tectonics, some examples from basin research show that positive inversion involvesflexural arching of prior lows or sedimentary basins and is not dependent on fault slip for its development (Harding, 1985; Corfield et al., 1996). In fact, some theoretical and case studies demonstrate that the reverse-sense fault displacement of major faults does not appear in mild-inversion, until all the normal-sense displacement is lost beyond the null-point in mod-erate-to-strong-inversion (Cooper and Williams, 1989; Song, 1997; Turner and Williams, 2004; Jackson et al., 2013). Therefore, the changes of structural relief from previous lows (outlined by thick deposits) to highs (outlined byflexural arching or uplift) indicates the structural in-version in seismic lines, even if without reverse-reactivation of pre-thrusting normal faults (e.g.Tavarnelli, 1996).
The recognition of the syn-kinematic strata in the basin infill, and the analysis of the interplay between stratigraphic horizon and faulting en-abled us to assign an age to fault activity in the Eastern Tyrrhenian Sea. Here we divided the fault structures into three groups according to their age.
5.1. Lower Pleistocene structures
Along the Campania margin the deposition of Unit A was mainly controlled by NW-trending graben and NE-SW transfer faults (Fig. 9). In the North there is a triangular basin (Northern Gaeta Bay), bounded by NNE-trending faults and NW-trending faults, with the apex located in the northern coast, and by an E-W transform fault (Milia et al., 2013). Unit Afilled also two thick and large NW-SE trending graben: the Campania Plain and Campi-Flegrei-Naples Bay basins (Milia et al., 2013; Milia, 1999; Milia and Torrente, 1999, 2015b). Two thickness maximaN1.9 s (twtt) are present in the Salerno Bay and Cilento basins (Fig. 9). The Salerno Bay Basin, covering an area of approximately 365 Km2, is characterized by an array of NW-trending normal faults that
forms graben structures (Fig. 5). These normal faults end abruptly to-ward northwest and southeast against NE-trending high-angle transfer faults (Figs. 5, 9). The Cilento Basin, located on the slope in the offshore of the Cilento Promontory, covers an area of approximately 345 Km2 and is NW-SE elongated (Fig. 9). Furthermore, other NW-trending nor-mal faults were active in the Campania Margin, offshore Capri Island
and in the Sorrento Peninsula, during the Lower Pleistocene (Milia and Torrente, 1997). A geological link between the Southern Campania Mar-gin (Cilento offshore) and bathyal Marsili basin is presented by the CROP M6B seismic profile (Fig. 13b). The oldest structures correspond to normal faults (F16, F15, F14) bounding the Tyrrhenian bathyal zone. The multibeam map of the Tyrrhenian Sea (Marani et al., 2004) re-veals that these structures trend NW-SE and correspond to the Sartori Lineament Auct. These boundary faults down throw the basement at a depthN5 s (approximately 3800 m) and are responsible for the crustal thinning linked to the formation of the bathyal region. The age of sedi-ments that onlap the acoustic basement and these faults has been cali-brated using the stratigraphy of the ODP Site 650 drilled in the Marsili basin, that is characterized, from older to younger, by 32 m of vesicular basalts overlain by a 602 m-thick succession of Pleistocene, where the oldest sediments are 2.0 Ma-old (biozone MPL6/NN18;Channell et al., 1990). Thus, these NW-SE faults (F16, F15, F14) date from the Lower Pleistocene.
Unlike the Campania Margin, in the Calabria Margin, the structures controlling the deposition of the Unit A (Fig. 9), are E-W trending syn-sedimentary faults (Milia et al., 2009) and pre-existing upper Miocene NNW-SSE trending normal faults (Milia and Torrente, 2014). Unit A is folded and presents in its upper part a syn-sedimentary wedge (Fig. 6). 3-D architecture of the folds and their relationships with the tectonically enhanced unconformities were reconstructed. The 3-D
model of the 1.0 surface shows (Fig. 6b) folds with en échelon pattern corresponding to large antiforms and a main synform that deepens in the central part of the basin. The en échelon fold pattern is consistent with the occurrence of a NW-SE sinistral shear zone (Milia et al., 2009). In conclusion, the Calabria Margin recorded a change in the tec-tonic regime from extension to strike slip regime.
5.2. Early Middle Pleistocene structures
The isochron map of the Unit B shows several thickness maxima (Fig. 11). On the base of the stratal architecture, we distinguished two basin styles: (i) basins characterized by low to moderate symmetrical subsidence, whose sediments prograde from the coast toward the sea filling the depocenters (Northern Gaeta Bay Basin, Salerno Bay Basin, Paola Basin; (Fig. 5); (ii) half graben basins featuring asymmetrical high subsidence controlled by NE-SW normal faults (Figs. 3, 4). This sec-ond category includes the Campi Flegrei-Naples Bay basin featuring basal angular unconformity and basin architecture that documents a block faulting coeval to the deposition of Unit B. Because the Campi Flegrei-Naples Bay basin is underfilled by Unit B sediments, it is easily identified in the contour map of horizon 0.7 (Fig. 14), but it is not evi-dent in the isochron map (Fig. 11). During this time, it started the for-mation of the Central Gaeta Bay and Campania Plain basins, Salerno Valley basin, Capri Basin, and Salerno-Cilento basin, with the deposition
Fig. 8. The 3-D digital model inserted into the spatial-oriented grid of the acoustic basement in the Eastern Tyrrhenian Margin. The view is from south-west and the vertical scale is in seconds.
of their sediments as syn-kinematics wedges. In particular the CROP M36 seismic profile displays (Fig. 13): (i) asymmetrical fault blocks dip-ping toward NW and bounded by NE trending normal faults (BFF, F8, F9); (ii) two structural highs bounding the Salerno Valley and the Saler-no-Cilento basins; (iii) a main angular unconformity between Lower and Middle Pleistocene deposits, marking the tilting of the fault blocks. It is worth noting that the activity of NE-trending faults during the de-position of the unit B was limited to the area between the Central Gaeta Bay Basin to the North and offshore Salerno bay to the South.
Prominent pop up and inversion structures were recognized off Na-ples Bay and in the Salerno Bay. Offshore NaNa-ples Bay, the interpretation of both multibeam bathymetry (Fig. 2) and CROP M30 seismic profile (Fig. 15) reveals a NW-SE, 40 km-long and 5 km-wide, pop-up structure (Sirene Seamount) bounded by high-angle reverse faults displacing Unit A. Its morphology displays an approximately 700 m high structural relief confined on both side by two symmetrical sedimentary basins [floored respectively, from NE to SW, at 1500 s (approximately 1000 m of water depth) and 1800 s (1350 m of water depth)],filled by Middle-Late Pleistocene deposits that cover unconformable the Lower Pleistocene deposits. According to the results of analogue models (Schellart and Nieuwland, 2003), we assume that the NW-SE Sirene Seamount pop up structure developed above a parallel basement strike slip fault. The NW-SE Sirene pop-up structure forms the western bound-ary of the Middle Pleistocene NE trending faults affecting the Campania margin. Because these NE-SW normal faults are absent in the contigu-ous Tyrrhenian bathyal zone, we interpret the Sirene structure as a transform fault developed at the boundary of the Middle Pleistocene SE-directed extension of the continental margin. In Salerno Bay the in-terpretation of seismic profile E-117 suggests that inversion structures formed after the sedimentation of the Unit A strata (Fig. 5). These strata, indeed, resulted folded, faulted and bounded at the top by an erosional surface covered by Unit B. Many folds and faults affect only the silico-clastic sedimentary succession (basinal Liguride units and Miocene clas-tic deposits) that covers the carbonaclas-tic basement, suggesting a Pleisto-cene reactivation of the oldest thrust surface. Therefore, during the
Fig. 10. Interpreted seismic section across the Salerno Valley basin. A = Unit A, B = Unit B, C = Unit C.
inversion tectonics, decoupling levels were activated in correspondence of the thrust fault and a disharmony between the stiff lower unit and the overlying thin-skinned covers unit was created. Similar structures have been documented in the Maracaibo basin (Roure et al., 1997).
It should be noted that the Salerno Bay basin changed from syn-rift to post-rift stage at lower/middle Pleistocene boundary, producing an angular unconformity. The undeformed prograding units (B and C Units) of the Salerno basin (seismic line E-204,Fig. 5) post-date the in-version tectonics and indicate a tectonic stability of the basin. 5.3. Late Middle Pleistocene-present structures
During this stage the extensional tectonics is only localized in the Campania Margin (Campania Plain, Southern Gaeta Bay and Sapri ba-sins;Fig. 13). The stratigraphic signature of this rifting episode in the Campania Plain corresponds to a basal angular unconformity and an abrupt increase in water depth. A very rapid subsidence of the basin permitted the deposition of c. 500 m of prodelta mudstones that cov-ered abruptly infra-littoral deposits (Cancello well,Milia and Torrente, 2015b). Voluminous volcanic activity and eastwards migration of fault-controlled asymmetrical subsidence from the Southern Gaeta Bay basin to the Campi Flegrei-Naples Bay basin occurred during the de-position of Unit C (Milia and Torrente, 2007, 2011; Torrente and Milia, 2013). The eastwards migration of extension is documented by a
geological section (Fig. 16), displaying two stages of tectonic subsi-dence: (i) normal faults active between 0.4 and 0.1–0.15 Ma in the Southern Gaeta Bay basin (Milia et al., 2013; Torrente and Milia, 2013); (ii) normal faults younger than 0.1 Ma at Campi Flegrei-Naples Bay basin (Milia, 2000; Bellucci et al., 2006; Milia et al., 2006; Torrente et al., 2010; Milia and Torrente, 2011). In the Bay of Naples the lower part of Unit C shows a rapid seawards progradation whereas its upper part is characterized by a thick volcanoclastic wedge deposited during a younger, post-0.1 Ma, tectonic subsidence stage (Figs. 4, 16) (Milia, 1999, 2000; Milia et al., 2006).
Offshore Cilento the entire stratigraphic succession is affected by folding and a wide deformation zone, approximately trending E-W, formed during Late Middle Pleistocene-Present (see map ofFig. 13). This deformation zone, going southwards corresponds to a restraining band and a releasing band of a strike-slip fault zone affecting the base-ment. In particular, folds andflexural arching, converging to a main ver-tical fault, visible in the CROP seismic profiles (Fig. 13a, central part of M36;Fig. 13b, northern part of M6B), can be linked to a positive struc-tural inversion, while a negativeflower structure occurs in the southern part of CROP seismic profile M36. Toward the South, in the Paola Basin, NW-SE dextral faults (see map ofFig. 13) were active and produced small pull apart basins (Milia et al., 2009). One of these features is im-aged at the southern end of the CROP M36 seismic profile (Fig. 13c). It displays a flexural arching converging toward the main fault,
corresponding to an anticlinorium crest, and striking different seismic features North and South of this fault. This overall geometry confirms a lateral movement of the fault blocks.
6. Discussion
Interpreting the data discussed above together with the results of re-cent models of seismic tomography carried out in the ETM, allows to an-alyze the kinematic link existing between the lower and upper plate. In this way, we get a complete picture of the tectonic evolution of the ETM, lacking so far, despite the large amount of seismic and geological studies carried out in this area in the lastfifteen years.
6.1. Seismic tomography models of the subduction zone
Knowledge of the mantle velocity structure beneath Apennine and Tyrrhenian Sea has been significantly improved in the last ten years using seismic tomography techniques. Despite intrinsic differences be-tween local and regional tomography (i.e., model parameterization,
ray tracing, approach to the inverse problem solution, and model reli-ability assessment), all recent models of the area are consistent on a large scale (e.g.,Lucente et al., 1999; Bijwaard and Spakman, 2000; Piromallo and Morelli, 2003; Montuori et al., 2007; Chiarabba et al., 2008; Di Stefano et al., 2009; Koulakov et al., 2009; Neri et al., 2009; Giacomuzzi et al., 2012). They show an active subduction process of the Ionian oceanic slab, dipping ~ 70 NW beneath the Calabrian arc and southern Tyrrhenian, down toN600 km. This process, associated with large positive velocities anomalies, is also confirmed by the distri-bution of the intermediate and deep seismicity. In contrast, strong neg-ative velocity anomalies, observed beneath the central and southern Apennines down to 200–250 km depth, together with the absence of in-termediate seismicity, indicate zones of unperturbed mantle, which has been interpreted as a gap in the subducted Adriatic slab.
The features discussed above are also visible in the recent regional seismic tomography model ofKoulakov et al. (2009), available from open sources (http://www.ivan-art.com/science/REGIONAL/index. html). The model covers an area within 30°N, 55°N; 5°W, 40°E and ex-tends to a depth of about 700 km. The initial data used by this model
included travel times of P and S body waves reported by the Internation-al SeismologicInternation-al Center (ISC, 2001). All seismic events in the time period from 1964 to 2001 were localized using the algorithm described in
Koulakov and Sobolev (2006), which has as a major advantage the de-tection and rejection of the outliers present in the ISC catalogue. The crustal corrections were computed using EuCRUST-07, which provides estimates of depths and velocities of the upper/lower crust (Tesauro et al., 2008). We can observe from cross-section 1 (Fig. 17), drawn per-pendicular to the strike of the belt, negative velocity anomalies (up to −3.5%) underlying the southern Apennines up to a depth of ~250 km. These anomalies indicate theflow of the asthenosphere between the lithosphere of the Adriatic plate, ~100 km thick, and the detached part of the slab, sinking up to the bottom of the transition zone, which are in turn identified by positive velocity anomalies up to ~2.5% (cross sec-tion 1,Fig. 17). Above the detached slab, beneath the Tyrrhenian Sea, there are strong negative anomalies (up to−3–4%), which have a ther-mal origin, being coincident with the location of Quaternary volcanos and areas of high heatflow (e.g.,Zito et al., 2003). These observations, together with estimates of low subcrustal velocities (e.g.,Di Stefano et al., 2009), shallow Moho depth (e.g.,Panza, 1984), and S-wave attenu-ation (e.g.,Marone et al., 2004) evidence significant crustal extension and asthenospheric upwelling.
The gap in the Adriatic slab tends to be progressively reduced to-ward the South and closes beneath the central part of the Calabrian Arc, where we can observe positive anomalies (up to ~ 2.5%), showing the active subduction of the Ionian slab (cross-section 2,Fig. 17). The
latter tends to narrow in its uppermost part (at a depth ~ 100 km) up tob100 km, where the positive anomalies are reduced to b1%. Even if the fading of the positive anomalies at the same depths of the width re-duction of the slab makes questionable its continuity, the occurrence of the earthquakes in this area confirms that the slab is undetached and subduction is still active. Southward the Calabrian Arc, beneath north-eastern Sicily, there are negative anomalies up to−2.5% down to ~ 250 km (cross-section 3,Fig. 17), indicating the presence of a gap wide ~ 100 km, which induces asthenospheric upwelling between the Ionian and the African slab.
The origin of the slab break-off beneath the central and southern Ap-ennines, which has likely caused magmas generation with transitional geochemical signatures, between arc type and ocean-island basalt (OIB) type, of the Quaternary Campania Province (Campi Flegrei, Vesu-vius) and Vulture (e.g.Peccerillo, 2005; De Astis et al., 2006; Rosenbaum et al., 2008), is still matter of debate. Some authors have hypothesized that the Adriatic slab during its progressive rollback has been fragmented by tears, which causedflow of asthenospheric materials (e.g.,Wortel and Spakman, 2000; Chiarabba et al., 2008; Di Stefano et al., 2009; Giacomuzzi et al., 2012). Using numerical models,Wortel and Spakman (2000)demonstrated that when a tear in the slab is gen-erated, the decreased slab width may cause sudden acceleration of the retreat and an ultrafast opening of the back-arc regions. Indeed, in the segment of the plate boundary where a tear initiates, the slab pull is not transferred to the lithosphere at the surface, but the weight of the slab is at least partially supported by the still continuous part of the
slab. Stress concentration near the tip of the tear causes its lateral migra-tion and velocity increase of the slab retreat (Wortel and Spakman, 2000). The development of slab tears is consequential to the formation of zone of weakness at the trench of the subduction zone, such as arrival of continental lithosphere after a period of oceanic lithosphere subduc-tion (Wortel and Spakman, 2000). Heterogeneities inherited from Me-sozoic tectonics along the passive margins of the Alpine Tethys ocean (European and African plates) and variations in the thickness and prop-erties of the Adriatic and Ionian subducting lithosphere may have in-duced changes in the velocities of the slab retreat along the length of the subduction system and thus promoted the generation of slab tears (e.g.,Di Stefano et al., 2009). According to other authors (e.g.,Lucente and Speranza, 2001), the detachment process has not been induced by
tear migration along the strike of the subduction zone, but is due to the entering of thick continental lithosphere in the trench and thus is spatially confined to the part of the subduction zone where the thicker lithosphere is present (i.e. the central-southern Apennines). However, the hypothesis of simultaneous slab detachment along a part of the plate boundary seems less likely, since the occurrence in a particular segment would activate the stress concentration mechanism and thus the tear migration (Wortel and Spakman, 2000). Furthermore, we should consider that 3D morphology of the slab beneath central and southern Apennines, constructed by projecting the positive seismic to-mography anomalies down to the bottom of the transition zone, shows the presence of multiple fragmentations at various depths (Rosenbaum et al., 2008). According to other seismic tomography
Fig. 15. Interpreted seismic section Crop M30 across the Northern Campania Margin. A = Unit A, B = Unit B, C = Unit C Mz = Mesozoic carbonates, Cz = Cenozoic terrigenous deposits. The redfilled arrows in the map are extension directions. The lower inset displays the stratal terminations of Quaternary units.
models, the Adriatic slab beneath the Neapolitan region is continuous, dividing the slab tear occurring beneath the Central Apennines from that present beneath the Southern Apennines (e.g.,De Gori et al., 2001; Chiarabba et al., 2008; Giacomuzzi et al., 2012). The configuration of the seismic anomalies and the results of anisotropic studies (e.g.,
Civello and Margheriti, 2004) lead some authors to hypothesize a 3D mantle circulation (toroidalflow) causing asthenospheric flow toward the wedge around the sinking Ionian slab (e.g.,Montuori et al., 2007; Faccenna et al., 2011). Therefore, the slab configuration in the Central Mediterranean is likely much more complex than that usually described in the numerical models.
6.2. Upper plate Quaternary evolution
The deformation phase of the ETM represents the last period of the Neogene Tyrrhenian Sea opening. Even if previous studies discussed the tectonic evolution of diverse sectors of the ETM, the tectonic history of the whole margin is still unclear. In this study, we describe all the phases of deformation of the ETM and contiguous Apennine belt. Qua-ternary extensional basins and transform faults developed along the ETM during several tectonic stages (each of which has a consistent kine-matic evolution). The ETM structural pattern includes: poliphased ex-tension and associated transform fault zones; changes in the direction
of extension; formation of thick sedimentary basins and uplift zones (Fig. 18).
During the Lower Pleistocene (2.0–1.0 Ma) extensional tectonics af-fected the entire ETM and the westernflank of the Apennines, creating several basins (Fig. 18a): NW-SE trending normal fault formed in the NCM (e.g. Northern Gaeta Bay and Campania Plain basins), SCM (Saler-no Bay, Cilento and Sele Plain basins), in Campania, and Basilicata (e.g. Diano and Auletta basins;Brancaccio et al., 1991; Barchi et al., 2007). NE-trending and E-W trending normal faults developed, in the Marsili and Paola basins, respectively. These three contemporaneous extension directions (NE–SW in the Campania margin, NW–SE in Marsili basin and N–S in CM) follow the double-saloon-door model by Martin (2006), characterized by a couple of oppositely propagating rifts and a coeval orthogonal third rift: the Campania margin and Marsili basin ap-proximately match with the two arc-parallel rifts and the Paola basin with the rift orthogonal to the subduction zone. During this stage rapid counterclockwise rotations affected the hanging wall of the active thrust sheets along the outer front of the Apennines (Mattei et al., 2004).
In the late Lower Pleistocene (1.0–0.7 Ma;Fig. 18b) there was a change in the structural pattern: extensional faults continued their ac-tivity in the Campania Margin, Marsili basin, and westernflank of the Apennines, while en échelon folds, linked to a NW-SE left-lateral trans-fer zone, formed in the CM. The activity of this transtrans-fer zone could be re-lated to a higher speed of the Marsili basin opening, compared to that of the ETM.
In the Early Middle Pleistocene (0.7–0.4-Ma) an abrupt change of di-rection of extension (from NE-SW to NW-SE) in the ETM occurred (Fig. 18c). This change can be linked to the change of direction of the tectonic transport (from NE to SE) of the southern Apennines thrust belt (Patacca et al., 1990). The extensional basins related to this stage correspond to underfilled half grabens of the Campania margin (e.g. Campania Plain and Campi Flegrei-Naples Bay basins; Salerno Valley and Salerno-Cilento basins). During that time, NW-SE left lateral strike slip faults in the Apennines (Catalano et al., 2004; Schiattarella et al., 2005) and NW-SE transfer zones in the ETM were active. These transfer zones were affected by uplifts and inversion structures. We assume that the region of NW-SE strike-slip faulting of the upper plate has external boundaries corresponding to a couple of lithospheric tears: Vulture vol-cano fault and Sirene Seamount fault. Notably, at 0.7 Ma started the Quaternary uplift of Calabria (Westaway, 1993).
In the Late Middle Pleistocene to Present (post 0.4 Ma) extensional basins (Southern Gaeta Bay, Campana Plain-Campi Flegrei and Sapri ba-sins) and transfer zone formed (Fig. 18d). Intense volcanism and east-wards migration of the extension was recorded in the NCM. NNE-trending normal faults developed in the ETM and Apennines (Vulture volcano,Schiattarella et al., 2005) and this extensional pattern is coher-ent with the migration of the Calabrian accretionary prism toward the E-SE. During this tectonic stage, a NW-SE transfer zone, featuring restraining and releasing bends, formed from the SCM to the CM at the southwest border of the ETM.
6.3. Kinematic link between upper and lower plates
Seismic tomography studies of Central Mediterranean revealed the existence of a gap in the structure of the subducted slab. According to
Lucente et al. (2006)slab windows opened north (southern Apennines) and south (Sicily Channel) of the Ionian slab.Govers and Wortel (2005)
instead reported two STEP faults at the northern and southern bound-aries of the Ionian slab. The occurrence of a scissor-type detachment
Fig. 18. Kinematics of the Tyrrhenian backarc basin during Quaternary, modified after
(STEP) in the northern boundary of the Ionian slab is documented by an approximately 250 km-long gap of the slab spanning from the Southern Apennines (Campania region) to the central part of Calabrian arc, where the slab is continuous.
A STEP fault has the potential to modify the surface topography on a kilometer scale, forming large sedimentary basins. Therefore,
combining the space-time evolution of sedimentary basins of the ETM and the slab geometries, imaged from seismic tomography, we can un-derstand the correlation between lower and upper plate deformation occurring in correspondence of the slab edge. To investigate the correla-tion between the evolucorrela-tion of sedimentary basins (timing and direccorrela-tion of extension) and occurrence of slab edge break-off and STEP
propagation, we integrate multiple features of the hinge zone deforma-tion of the upper plate (such us stratigraphic signature of tectonics, fault pattern, chronology of basin evolution), as well as the interpretations of seismic tomography data. We found a direct correspondence between lower plate rupture propagation and upper plate deformation, suggest-ing a genetic link between upper and lower plates.
We propose a possible geodynamic scenario characterized by a slab detachment linked to a STEP propagation (Figs. 18, 19).
a) During the Lower Pleistocene (2.0–1.0 Ma) the upper plate exten-sion is directed perpendicular to the Apenninic trench (E-NE-direct-ed) contemporaneously, in the southern part, the extension is directed perpendicular to the Ionian trench (SE-directed). The diver-gence in the direction of extension between these two regions im-plies the opening of an external basin between them (Paola Basin). The difference in the trench orientation can be due to the different velocity in the rollback of the continental and oceanic slab sectors. b) Through the late Lower Pleistocene (1.0–0.7 Ma) the reduction in
the extension value along the Campania Margin and the increase in the extension value of the Marsili basin, induce the formation of a transfer fault zone in the Calabria Margin. These features can be interpreted as the response in the upper plate of the STEP fault nu-cleation and rollback of the southern slab.
c) The Early Middle Pleistocene phase (0.7–0.4 Ma) was a period of plates re-organization that recorded: (i) abrupt change in the exten-sion direction (from NE to SE), associated to the formation of rapidly evolving half graben localized in the Northern Campania Margin; (ii) end of the NE-directed thrust propagation in the Apennines and rapid uplift of Calabria. We should note that the deformation zone is localized in the ETM and bounded by two transfer zones toward the bathyal basins of the backarc and the external zone of the Apen-nines. The narrow belt of deformation is localized in correspondence of the northern boundary of the Ionian slab and can be interpreted as the response of the STEP fault propagation. The toroidalflow devel-opment around the northern edge of the Ionian and African slab (e.g.,Montuori et al., 2007; Faccenna et al., 2011) would explain the upwelling of asthenospheric material beneath Calabria, and its rapid uplift (Gvirtzman and Nur, 1999).
d) The Late Middle Pleistocene-Present (post-0.4 Ma) period records a minor reorientation (E-SE) of the direction of extension in the ETM and the development of a restraining bend in the SCM. The NCM was characterized by large-volume volcanic eruptions and an in-crease of subsidence and extension associated to high crustal thin-ning (10 km at Campi Flegrei). This deformation stage indicates the enlargement of the gap effect of the continuum propagation of the STEP fault.
7. Conclusions
Based on seismic tomography studies, a general consensus exists on the gaps in the Adriatic slab, which induced the upwelling of hot as-thenospheric material and generation of magmas with transitional geo-chemical signatures, between arc type and OIB type, of the Quaternary Campania Province (Campi Flegrei, Vesuvius) and Vulture volcanic complex (e.g.Peccerillo, 2005; De Astis et al., 2006; Rosenbaum et al., 2008). Several models of slab break-off have been proposed, which can be ascribed to two main categories, slab window and slab tear or STEP fault (e.g.Govers and Wortel, 2005; Rosenbaum et al., 2008). How-ever, despite all the previous reconstructions of the complex geometry and boundaries of the Adriatic slab, age and mode of formation and evo-lution of the slab break-off remain unclear, without the interpretation of the kinematics of the upper plate. Indeed, the tectonic history of the deep and rapidly subsiding basins formed along the ETM allows to un-derstand the mode of the interplay between lower and upper plate.
In this study, we combined the analysis of deep and shallow process-es, mainly focusing on the relationship between the evolution of the lower plate (subduction slab) and upper plate (backarc sedimentary ba-sins) and the resulting geodynamic implications. Our kinematic analysis reveals a WNW-trending deformation belt bounded by transfer faults, corresponding to the ETM, characterized by backarc opening with NE-directed extension in the Lower Pleistocene (2–0–0.7 Ma) followed by SE-directed extension in the Middle Pleistocene (post-0.7 Ma). This abrupt change of ~90° in the extension direction of the ETM during Qua-ternary is inconsistent with the features predicted by the current models of backarc opening. Therefore, we hypothesized the existence of a STEP fault currently bounding the northern margin of the Ionian plate. This STEP nucleated about 1 Ma ago along the ETM, forming a gap in the Adriatic slab, which propagated for about 250 km southward reaching the central part of the Calabrian arc, where the slab is presently continuous. Notably, the deformation belt is bounded by lithospheric faults, which has favored recent volcanic activities, controlling magma rise from deep reservoirs to surface.
Acknowledgements
This study was funded by Università del Sannio (FRA 2015), Utrecht University, and the Netherlands Research Centre for Integrated Solid Earth Science (ISES-2014-UU-08, ISES-2016-UUU-19). We thank the IHS Inc. that provided the Kingdom software. We are grateful to E. Tavarnelli and G. Tari for the detailed review and highly stimulating comments. We acknowledge S. Cloetingh for his suggestions on an ear-lier version of the manuscript.
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